A Universe that expands and cools today, like
ours does, must have been hotter and denser in
the past.

Initially, the Big Bang was regarded as the
singularity from which this ultimate, hot, dense
state emerged.

But we know better today.

The Big Bang
Wasn't The Beginning - After All Starts With A Bang
The Universe began not with a whimper, but with a bang!

At least, that's what
you're commonly told:

the Universe and
everything in it came into existence at the moment of the Big
Bang.

Space, time, and all the
matter and energy within began from a singular point, and then
expanded and cooled, giving rise over billions of years to the
atoms, stars, galaxies, and clusters of galaxies spread out across
the billions of light years that make up our observable Universe.

It's a compelling,
beautiful picture that explains so much of what we see, from the
present large-scale structure of the Universe's two trillion
galaxies to the leftover glow of radiation permeating all of
existence.

Unfortunately, it's also
wrong, and scientists have known this for almost 40
years...

Vesto Slipher, (1917)

Proc.
Amer. Phil. Soc., 56, 403

First noted by Vesto Slipher, the more distant a
galaxy is, on average, the faster it's observed
to recede away from us.

For years, this defied explanation, until
Hubble's observations allowed us to put the
pieces together: the Universe was expanding.

The idea of the Big Bang first came about back
in the 1920s and 1930s.

When we looked out at
distant galaxies, we discovered something peculiar:

the farther away from
us they were, the faster they appeared to be receding from us.

According to the
predictions of Einstein's General Relativity, a static
Universe would be gravitationally unstable; everything needed to
either be moving away from one another or collapsing towards one
another if the fabric of space obeyed his laws.

The observation of this
apparent recession taught us that the Universe was expanding today,
and if things are getting farther apart as time goes on, it means
they were closer together in the distant past.

NASA / STScI / A. Felid

If you look farther and farther away, you also
look farther and farther into the past.

The earlier you go, the hotter and denser, as
well as less-evolved, the Universe turns out to
be.

An expanding Universe doesn't just mean that things get
farther apart as time goes on, it also means that the light existing
in the Universe stretches in wavelength as we travel forward in
time.

Since wavelength
determines energy (shorter is more energetic), that means the
Universe cools as we age, and hence things were hotter in the past.
Extrapolate this back far enough, and you'll come to a time where
everything was so hot that not even neutral atoms could form.

If this picture were
correct, we should see a leftover glow of radiation today, in all
directions, that had cooled to just a few degrees above absolute
zero.

The discovery of this
Cosmic Microwave Background in 1964
by Arno Penzias and Bob Wilson was a breathtaking
confirmation of the Big Bang...

NASA / WMAP Science Team

According to the original observations of
Penzias and Wilson, the galactic plane emitted
some astrophysical sources of radiation
(center), but above and below, all that remained
was a near-perfect, uniform background of
radiation.

It's tempting, therefore, to keep extrapolating backwards in time,
to when the Universe was even hotter, denser, and more compact.

If you continue to go
back, you'll find:

A time where it
was too hot to form atomic nuclei, where the radiation was
so hot that any bound protons-and-neutrons would be blasted
apart.

A time where
matter and antimatter pairs could spontaneously form, as the
Universe is so energetic that pairs of
particles/antiparticles can spontaneously be created.

A time where
individual protons and neutrons break down into a
quark-gluon plasma, as the temperatures and densities are so
high that the Universe becomes denser than the inside of an
atomic nucleus.

And finally, a
time where the density and temperature rise to infinite
values, as all the matter and energy in the Universe are
contained within a single point: a singularity.

This very final point -
this singularitythat represents where the laws of physics break down - also
is understood to represent the origin of space and time.

This was the ultimate
idea of the Big Bang...

NASA / CXC / M.Weiss

If we extrapolate all the way back, we get to
earlier, hotter, and denser states.

Does this culminate in a singularity, where the
laws of physics themselves break down?

Of course, everything
except that last point has been confirmed to be true!

We've created
quark-gluon plasmas in the lab

We've created
matter-antimatter pairs

We've done the
calculations for which light elements should form and in
what abundances during the early stages of the Universe,
made the measurements,

...and found that they
match with the Big Bang's predictions.

Coming forward even
farther, we've measured the fluctuations in the cosmic microwave
background and seen how gravitationally bound structures like stars
and galaxies form and grow.

Everywhere we look, we
find a tremendous agreement between theory and observation.

The Big Bang looks like a
'winner'...

Chris Blake and Sam Moorfield

The density fluctuations in the cosmic microwave
background provide the seeds for modern cosmic
structure to form, including stars, galaxies,
clusters of galaxies, filaments, and large-scale
cosmic voids.

Except, that is, in a few regards.

Three specific things you
would expect from the Big Bang didn't happen.

In particular:

The Universe
doesn't have different temperatures in different directions,
even though an area billions of light-years away in one
direction never had time (since the Big Bang) to interact
with or exchange information with an area billions of
light-years in the opposite direction.

The Universe
doesn't have a measurable spatial curvature that's different
from zero, even though a Universe that's perfectly spatially
flat requires a perfect balance between the initial
expansion and the matter-and-radiation density.

The Universe
doesn't have any leftover ultra-high-energy relics from the
earliest times, even though the temperatures that would
create these relics should have existed if the Universe were
arbitrarily hot.

Theorists thinking about
these problems started thinking of alternatives to a
"singularity" to the Big Bang, and rather of what could recreate
that hot, dense, expanding, cooling state while avoiding these
problems.

There's always a non-zero probability that
inflation will end (denoted by a red 'X') at any
time, giving rise to a hot, dense state where
the Universe is full of matter and radiation.

But in regions where it doesn't end, space
continues to inflate.

Instead of an
arbitrarily hot, dense state, the Universe could have begun from
a state where there was no matter, no radiation, no antimatter, no
neutrinos, and no particles at all.

All the energy present in
the Universe would rather be bound up in the fabric of space
itself:

a form of vacuum
energy, which causes the Universe to expand at an exponential
rate.

In this cosmic state,
quantum fluctuations would still exist, and so as space expanded,
these fluctuations would get stretched across the Universe, creating
regions with slightly-more or slightly-less than average energy
densities.

And finally, when this
phase of the Universe - this
period of inflation - came to an
end, that energy would get converted into matter-and-radiation,
creating the hot, dense state synonymous with the Big Bang.

E. Siegel, with images derived from

ESA/Planck
and the DoE/NASA/ NSF interagency task force

on CMB
research

The quantum fluctuations inherent to space,
stretched across the Universe during cosmic
inflation, gave rise to the density fluctuations
imprinted in the cosmic microwave background,
which in turn gave rise to the stars, galaxies,
and other large-scale structure in the Universe
today.

This was regarded as a
compelling-but-speculative idea, but there was a way to test it.

If we were able to
measure the fluctuations in the Big Bang's leftover glow, and they
exhibited a particular pattern consistent with inflation's
predictions, that would be a "smoking gun" for inflation.

Furthermore, those
fluctuations would have to be very small in magnitude:

small enough that the
Universe could never have reached the temperatures necessary to
create high-energy relics, and much smaller than the
temperatures and densities where space and time would appear to
emerge from a singularity.

In the 1990s, 2000s, and
then again in the 2010s, we measured those fluctuations in detail,
and found exactly that.

NASA / WMAP science team

The fluctuations in the cosmic microwave
background, as measured by COBE (on large
scales), WMAP (on intermediate scales), and
Planck (on small scales), are all consistent
with not only arising from a scale-invariant set
of quantum fluctuations, but of being so low in
magnitude that they could not possibly have
arisen from an arbitrarily hot, dense state.

The conclusion was
inescapable:

the hot Big Bang
definitely happened, but doesn't extend to go all the way back
to an arbitrarily hot and dense state.

Instead, the very early
Universe underwent a period of time where all of the energy that
would go into the matter and radiation present today was instead
bound up in the fabric of space itself.

That period, known as
cosmic inflation, came to an end and gave rise to the hot Big
Bang, but never created an arbitrarily hot, dense state, nor did
it create a 'singularity.'

What happened prior to
inflation - or whether inflation was eternal to the past - is still
an open question, but one thing is for certain: